Kin selection and parasite evolution: Higher and lower virulence with hardand soft selection

Conventional models predict that low genetic relatedness among parasites th at coinfect the sa,ne host leads to the evolution of high parasite virulenc e. Such models assume adaptive, responses to hard selection only. We show t hat if soft selection is allowed to operate, low relatedness leads instead to the evolution of low virulence. With both hard and soft selection, low r elatedness increases the conflict among coinfecting parasites. Although par asites can only respond to hard selection by evolving higher virulence and overexploiting their host, they can respond to soft selection ip evolving o ther adaptations, such as interference, that prevent overexploitation. Be c ause interference can entail a cost, the host may actually De underexploite d, and virulence will decrease as a result of soft selection. Our analysis also shows that responses to soft selection can have a much stronger effect than, responses to hard selection. After hard selection has raised virulen ce to a level that is an evolutionarily stable strategy, the population, as expected, cannot De invaded by more virulent phenotypes that respond only to hard selection. The population remains susceptible to invasion by a less virulent phenotype that responds to soft selection, however.